This invention relates generally to gas turbine engine combustion systems, and more particularly, to methods and apparatus for controlling the operation of the combustion systems.
Gas turbine engines typically include a compressor section, a combustor section, and at least one turbine section. The compressor discharge air is channeled into the combustor where fuel is injected, mixed and burned. The combustion gases are then channeled to the turbine which extracts energy from the combustion gases.
Gas turbine engine combustion systems must operate over a wide range of flow, pressure temperature and fuel/air ratio operating conditions. Controlling combustor performance is required to achieve and maintain satisfactory overall gas turbine engine operation and to achieve acceptable emissions levels, the main concern being NOx and CO levels.
One class of gas turbine combustors achieve low NOx emissions levels by employing lean premixed combustion wherein the fuel and an excess of air that is required to burn all the fuel are mixed prior to combustion to control and limit thermal NOx production. This class of combustors, often referred to as Dry Low NOx (DLN) combustors are usually limited by pressure oscillations known as “dynamics” in regards to their ability to accommodate different fuels. This is due to the change in pressure ratio of the injection system that results from changes in the volumetric fuel flow required. This constraint is captured by the Modified Wobbe Index; i.e., the combustion system will have a design Wobbe number for optimum dynamics. The Modified Wobbe Index (MWI) is proportional to the lower heating value in units of BTU/scf and inversely proportional to the square root of the product of the specific gravity of the fuel relative to air and the fuel temperature in degrees Rankine.
This problem as so far been addressed by restricting changes in Wobbe number or by adjusting the fuel temperature to recenter the Wobbe number of a given fuel. Fuel split changes to the combustor (e.g., retuning) are also possible, but they may impact emissions.
Such systems often require multiple independently controlled fuel injection points or fuel nozzles in each of one or more substantially parallel and identical combustors to allow gas turbine operation from start-up through full load. Furthermore, such DLN combustion systems often function well only over a relatively narrow range of fuel injector pressure ratios. The pressure ratio is a function of fuel flow rate, fuel passage flow area and gas turbine cycle pressures before and after the fuel nozzles. Such pressure ratio limits are managed by selection of the correct fuel nozzle passage areas and regulation of the fuel flows to the several fuel nozzle groups. The correct fuel nozzle passage areas are based on the actual fuel properties which are nominally assumed to be contact.
Historically, pipeline natural gas composition in general, and specifically its Modified Wobbe Index, have varied only slightly. Fuel nozzle gas areas are sized for a limited range of fuel MWI, typically less than about plus or minus five percent of the design value, and for a gas turbine with Dry Low NOx combustion systems with multiple fuel injection points, the gas turbine combustion system is set up with fuel distribution schedules such that the fuel splits among the various injection points vary with machine operating conditions. For some DLN combustion systems, if fuel properties change by a value of more than about plus or minus two percent in MWI, it is necessary to make fuel schedule adjustments while monitoring both emissions and combustion dynamics levels. Such fuel schedule adjustment is called “tuning”, a process that requires technicians to set up special instrumentation, and that may take a day or longer to accomplish. Furthermore, when the fuel supplied to a specific gas turbine installation is from more than one source (with different compositions and resulting MWI), it has been necessary to “retune” the fuel split schedules (and, prior to the invention disclosed herein) repeat for every fuel supply switch. Furthermore, any blend of the two or more fuels is the equivalent of another fuel composition and as a result, a variable blend of the fuels that exceeds the MWI range of the combustor design could not be tolerated prior to this invention without operational adjustments to the gas turbine and/or gas turbine combustor (e.g. variable fuel temperature). Gas turbine engine efficiency can be improved by employing an available source of heat such as low energy steam or water to preheat the fuel gas entering the gas turbine combustor. For gas turbines employing heated gas, load up time may depend on the time required to generate hot water in the initially cool heat recovery steam generator to heat the fuel gas to a minimum required level. Until the fuel gas reaches the required temperature and consequently the required MWI, some combustor designs are unable to operate in the low NOx combustion mode. If the minimum acceptable gas temperature level can be reduced, which corresponds to raising the maximum permissible MWI value, gas turbine operations are improved and total emissions reduced by shortened load up times.
Operation outside of the design MWI range can for some of DLN combustion system designs result in combustion dynamics levels (noise due to oscillatory combustion process) that are large enough to shorten the maintenance intervals or even cause hardware damage and forced outages. It is desirable therefore to permit a larger variation in gas fuel composition, temperature and resulting MWI, while maintaining low emissions and combustion dynamics levels within predetermined limits.